PROJECT SUMMARY We study formation of upper gastrointestinal premalignant lesions, as well as their progression to neoplasia. A key premalignant lesion in gastric cancer, pseudopyloric or Spasmolytic Polypeptide Expressing Metaplasia (SPEM), occurs when chronic inflammation (usually via the bacterium Helicobacter pylori) leads to death (atrophy) of acid-secreting parietal cells and a change in differentiation pattern of the other key gastric gland lineage, the zymogenic chief cell (ZC). We and others have shown that ZCs in SPEM become proliferative and metaplastic. They do so via a sequence of molecular-cellular events that is conserved across many tissues and species in scenarios where mature cells are recruited back into the cell cycle in response to tissue damage. Thus, ZCs undergoing metaplasia undergo an evolutionarily conserved program, termed paligenosis. In paligenosis ZCs: first degrade/recycle their differentiated cell specialized components (Stage 1), then induce expression of more progenitor-like genes (eg. Sox9 = Stage 2), and ultimately re-enter the cell cycle (Stage 3). Metaplasia can resolve as tissue is repaired or become chronic and increase risk for progression to dysplasia and cancer. We have shown that paligenosis is governed by dynamic changes in mTORC1, the key cellular translational control complex. mTORC1 is elevated at baseline in ZCs to drive production of digestive enzymes; it shuts off at Stage 1 and reactivates at Stage 3. Without mTORC1, paligenosis stops at Stage 2 with cells looking metaplastic but unable to enter the S-phase. Here, we explore the mechanisms underlying progression through paligenosis. Our overarching hypothesis is that paligenosis is a licensing procedure to ensure that old cells, which may have accumulated mutations and have damaged organelles, undergo a strict error-checking protocol before being allowed to reenter the cell cycle. Errors in paligenosis may lead to tumors as cells with mutations may inappropriately proliferate. In the current proposal, we focus on mechanisms regulating Stage 1 and Stage 3 and determine effects on tumorigenesis with correlation to humans. We undertake these studies with our long-standing collaborators at China Medical University in Shenyang, China. They provide vast tissue databanks of normal, metaplastic, and cancerous stomach tissue with accompanying clinical data. And they have established pipelines for sequencing and bioinformatic analysis. Our Aims are: 1) to determine effects of cells either being stuck or skipping Stage 1; 2) to elucidate the effects of altering the p53-mTORC1 hub that determines Stage 3; and 3) to translate our findings towards human relevance using long-term tumorigenesis models with mouse mutants in cycles of paligenosis as well as correlation in mouse/human ex vivo organoids and in tissue microarrays of ~1000 human patients. Together, our combined US-China teams will bring resources and expertise to bear on a global, understudied health problem with particularly high prevalence in Northeastern China: gastric cancer.